Touch screen systems that allow users to interact directly with content displayed on a 2D surface are becoming ubiquitous in many computing platforms. However, several limiting factors hinder the usability of these systems. First, few of these systems can preserve the familiar mechanical response of traditional tools such as brushes, pens, and bare fingers. Second, there are even fewer systems that can simultaneously detect multiple interaction points and are therefore unable to detect the size, shape, and other nuances of the drawing/writing tool. Third, most touch screen technologies don't scale gracefully to large surfaces, or involve significant expense or other practical limitations when extended to perimeters beyond a meter or so. Finally, most of these systems require severe modification of the drawing surfaces, or that bulky devices or materials be attached to the sensing side of the drawing surface, which means that the implementation of a large, comfortable drawing surface is simply not practical, robust, or cost-effective.
U.S. Pat. No. 4,254,33 issued to Arne Bergstrom on Mar. 3, 1981 describes the use of a touch key or panel constructed of an optically dense body, such as glass, which transmits light by “total internal reflection” from a light source, such as a light emitting diode (LED) to a light detector. When an object is brought into contact with the light-transmitting body, some of the transmitted light is refracted outward, resulting in detected attenuation of the light received by the detector. Bergstrom further suggests that this design principle can be used to a large touch keyboard in which the radiation field inside a large, plane-parallel glass plate forms a light-beam matrix, invisible from the outside, the rows and columns of the matrix being formed by a number of pairs of light source/detector pairs, with each source directing light towards an edge of the plate and transmitting light through the edges via multiple total internal reflection between the upper and lower surfaces of the plate to the corresponding detector on the opposite side of the plate. Touching the upper surface of the plate at the intersection between a row and column beam reduces the total internal reflection of these two intersecting beams, generating a key press identification.
Unfortunately, the touch keyboard proposed by Bergstrom can be implemented only with difficulty. A large number of light sources (one for each light detector) is required, and each light source must either emit light that is focused on a single detector, or each detector must be focused on a single source. This constraint makes the use of light source/detector pairs impractical for higher resolution devices which must necessarily use a large number of closely spaced detectors and sources.
There is accordingly a need for a practical, low-cost, high-resolution touch responsive system capable of determining of the position, size and/or shape of an area of contact on a large surface area.